Scanning-type projection-exposure apparatus are disclosed in, for example, Japanese laid-open patent application no. 7-57986. The projection-exposure apparatus disclosed in the JP '986 application includes a projection-optical system composed of plural erecting projection-optical units having a magnification of 1. An erecting projection-optical unit consists of two optical groups, each including a dioptric system and a concave reflecting mirror. A mask and a substrate are moved together relative to the projection optical system, thereby effecting a scanning exposure of the substrate with the pattern defined by the mask. Also, the positional relationship between (i.e., the distance between) the mask and the substrate must be maintained constant during the scanning exposure.
Unfortunately, it is difficult to maintain constancy of the positional relationship between the mask and the substrate during actual scanning. A changing distance between the mask and the substrate during scanning can arise from, for example, mechanical distortion of the mask and the substrate caused by the actual scanning movement.
In view of the foregoing, there is a need for a way to maintain a constant positional relationship between the mask and the substrate during a scanning projection exposure so as to produce a high-quality projected image over the entire exposure area of the substrate.